Dr. John Rabun, a general and forensic psychiatrist at Clayton Behavioral Health, says in circumstances like the Connecticut school shooting, it is likely the shooter had an issue with his mother, or quite possibly something that took place at the school. But he acknowledged that this is speculation.

He says mass homicides in the workplace or at schools are increasing and cited the Texas tower shootings in the 1960s as one of the first. The FBI defines a mass shooting as involving four or more victims in one location

When asked if these events are all, in one way or another, copy-cats of each other, he says individuals don't act until they have thought extensively about acting. Often they harbor some deep grudge and anger about a particular person and place.

"So in that regard they're not a copy-cat. Do they get ideas about making a final statement in their life to this level? Yes they do. They do get that idea. They do read about Columbine, they do read about other mass shooters, but it still takes that extra anger fueled with whatever grudge they hold to force someone to act in this manner," said Dr. Rabun.

The suspect in the Connecticut school shooting has been identified as 20-year-old Adam Lanza, and it seems in these cases the suspect is often times a man in his early 20s.

"Because the younger you are the less capable you are of deferring and thinking about consequence; deferring your behavior, thinking about consequences. As you get older you mature, you reflect more, you understand the consequence of what's going to occur, you act in a certain manner. You think before you act," said Dr. Rabun.

Dr. Rabun also pointed out these cases frequently involve undiagnosed or untreated mental illness and often times they involve a degree of calculation, planning, and premeditation.

He says the perpetrators may even perform what psychiatrists refer to as behavior trials or tryouts, where they go through the motions and evaluate whether they can get away with acting.